Method of forming insulating coat on steel and composition therefor



Ilnitcl States Patent METHOD OF FORMING INSULATING COAT ON STEEL AND COMPOSITION THEREFOR Paul E. Perry, Tarentum, Pa., assignor to Allegheny Ludlum Steel tilorporation, Brackenridge, Pa., :1 c0rporation of Pennsylvania No Drawing. Application July 27, 1953, Serial No. 370,609

8 Claims. (Cl. 148--6.16)

This invention relates to insulating coatings for sheet and strip steel used for electrical applications and particularly, to an inorganic coating which may be applied before stamping or rolling operations in making electrical parts.

Prior to my present invention it has been common practice to apply insulating coatings which are usually referred to as core plates, to electrical steel for the purpose of minimizing eddy currents on steel parts used in motors, transformers, dynamos, and other electrical appa ratus. Coatings for this purpose have been either organic or inorganic in nature. The organic coatings con sisting of resins, varnishes or lacquers are applied as a final operation before assembling and after all heat treatments have been effected. Inorganic coatings have been made up of numerous materials, such as sodium silicate, talc, magnesium oxide, borax, etc., of a refractory nature; some contain phosphates.

I have determined that a good insulating coating for electrical steel should be insoluble in Water, not affected by oils used in transformers, have good adhesion, and possess lubricating qualities that will improve die life when the coated material is subjected to stamping operations. To the best of my knowledge, no known coating meets all these requirements.

This is a continuation-in-part of my application Serial No. 207,657, entitled, Insulating Coating for Electrical Steel, filed January 24, 1951, now abandoned.

It has thus been an object of my invention to provide an improved insulating coating for electrical steels;

Another object has been to provide a coating which will solve the problems heretofore presented in this connection and particularly, which will meet the factors above set forth;

A further object has been to devise an inorganic phosphate type of coating which may be applied as a solution to the metal and then baked thereon as a highly adherent improved coating;

These and many other objects of my invention will appear from the present disclosure.

In carrying out my invention, I provide an insulating coating containing phosphates with good insulating qualities, that has a minimum of dusting tendencies, that does not decompose under heat up to 1600 F., and that has definitely good lubricating properties during stamping operations. I employ three basic ingredients, namely: chromic acid, CrOs, mono basic ammonium phosphate, NH4H2PO4, and primary magnesium phosphate, MgH4(PO4.)2, as a water soluble solution film. This solution may be applied by spraying, dipping or rolling techniques and after its application, the phosphating is effected by baking the coated metal at a temperature of about 8001000 F. The solution provides a balanced film of chemicals which is applied as a water soluble layer and which does not become a chemically adherent film until reacted at a proper baking temperature. This solution, its application, and baking provide a coating film of superior insulating properties that has good penetration and is inherently bonded upon the base material, so that it is diflicult to remove by any normal amount of bending or flexing.

My phosphate coating method must not be confused with the industrial phosphatization processes for rustproofing iron and steel in phosphoric containing solutions. In these procedures, the phosphating, except for subsequent rinsing operations, is completed essentially when the metal is sprayed with or dipped into the solution. Coating is achieved by an ion exchange or chemical reaction between the metal and the phosphating solution. To obtain maximum efficiency in such baths, continuous chemical control tests are necessary for proper mainte nance of solution balance. Metal immersion time and temperature directly influence the qualtity of the coating. However, with silicon electrical steel, I have not found any of these processes satisfactory, principally because such solutions are only slightly reactive to such metal. I have been unable to successfully form electrostatically deposited phosphate films on electrical steel. Such coatings adhered poorly and had unsatisfactory insulation values.

Maintenance and application of my phosphating solution is much simpler than that required for other methods. Careful temperature reaction time and chemical solution control are not required. When applied, the solution may be used at any reasonable temperature, although some recrystallization of chemicals may occur if the temperature falls below about 70 F. No break-in period of adjustment procedure is required before a satisfactory metal coating can be obtained.

I have also determined that the composition of the coating solution will depend upon whether or not it is to be applied by dip or spray methods or by a rolling application. More concentrated solutions tend to compensate for the wiping action on the rolls and still provide the desired coating film.

The ingredients of my solution are in combination, water soluble, and dissolve in water to make a clear solution. Of the three basic ingredients, primary magnesium phosphate is the least soluble and serves principally as a filler to provide inorganic refractory residue. However, its acid content does make it somewhat reactive toward coated metal at elevated temperatures. Mono basic ammonium phosphate improves initial adhesion of the coating solution and at baking temperature, supplies a binding matrix which results from its reaction with the metal surface. The third basic component, chromic acid anhydride, acts as an inhibitor and oxidizing agent. The latter two chemicals have been found necessary in preparing a solution of magnesium phosphate; they improve its solution rate and provide a proper balance of adhesion and insulation of the coating as baked. The use of a small quantity of a non-ionic wetting agent such as soap (e. g. sodium lauryl sulfate) has been found beneficial for the coating application.

I have found that chromic acid and .mono basic ammonium phosphate increase the solubility of the primary magnesium phosphate and that chrornio acid is the more efiective of the two. The solubility of a chemically pure primary magnesium phosphate in water at 70 F. is approximately 0.5%, while that of the technical product is nearly sixteen times greater or about 8%. I find that the higher acid content of the technical product is responsible for this deviation. Such solubility variance remains similar at increased solution temperatures. However, regardless of the greater solubility of the technical compound, I have determined that the other two solute or dissolving chemicals are necessary for the dissolution of either technical or chemically pure primary magnesium phosphate. If these two other solution chemicals are applied individually to dissolve chemically pure primary J? magnesium phosphate, chromic acid is the more effective. For example, the acid is approximately sixty times more effective in a preparation of 25 parts by weight of primary magnesium phosphate solute in 100 parts by weight of water solvent; 1 part by weight of chrome acid solute in the 100 parts by weight of water solvent is required for this concentration. By increasing the chromic acid from 1 to 2 parts by weight in the 100 parts by weight of water solvent, it extends the primary magnesium phosphate solution possibility to nearly 45 parts by weight to the 100 parts by weight of water solvent. Approximately 24 parts by weight of mono basic ammonium phos phate solute in 100 parts by weight of water solvent is required to dissolve about 10 parts by weight of chemically pure primary magnesium phosphate solute in the 100 parts by weight of water solvent. It was observed that solutions containing no chromic acid were visibly reactive towards immersed metal.

Although I have discovered that proportionately greater concentrations with either salt are possible at elevated temperatures, fortunately such higher concentrations are not generally required, and adequate coatings have been obtained with concentrations possible in the solubility range near room temperature.

I also investigated weight variations of primary magnesium phosphate and mono basic ammonium phosphate to determine the effect on coating adhesion and dusting. Concentrations of primary magnesium phosphate of 10, 15 and 20 parts by weight as solutes in 100 parts by weight of water solvent, each containing 1 part by weight 9 of chromic and anhydride solute in the 100 parts by weight of water solvent were tested. The quantity of the mono basic ammonium phosphate in these three solution concentrations was increased in parts by weight of solute additions (from zero to 40 parts by weight) in 100 parts by weight of water solvent. The effectiveness of each weight variation was recorded as the results of tests for insulation value on the coated baked panels. For each of the three quantities of primary magnesium phosphate, a graph Was prepared with these values. On the curves, a range of about 20 to 40 parts by weight of mono basic ammonium phosphate solute in 100 parts by weight of Water solvent with 10 to parts by Weight of primary magnesium phosphate solute in the 100 parts by weight of water solvent gave thebest insulation results. Also coatings prepared from parts by weight of primary magnesium phosphate solute in 100 parts by weight of water solvent with about parts by weight of mono basic ammonium phosphate solute in the 100 parts by weight of water solvent gave similar good results. Further increasing the mono basic ammonium phosphate While employing the same amount of the chromic acid anhydride produced coatings that blistcred. During these experiments, a dipping procedure was used.

I have determined from insulation tests, such as above outlined, that better coating films can be obtained With a dip coating application from solutions that have from about 10 parts to 25 parts by weight of primary magnesium phosphate solute in 100 parts by weight of water solvent, about 1 part by weight of chromic acid anhydride solute in the 100 parts by weight of water solvent, and about 20 to parts by weight of mono basic ammonium phosphate in the parts by weight of water solvent.

The optimum coating mixture (containing 100 parts by weight of water solvent) that provided a satisfactory insulation with a minimum of dusting when applied by other than a rolling method contains: about 10 parts primary magnesium phosphate solute by weight, about 20 parts mono basic ammonium phosphate solute by weight, and about 1 part chromic acid anhydride solute by weight. A total of 31 parts by weight of solute in 100 parts by weight of the aqueous solvent or 0.31 total parts by Weight in one part by weight of the aqueous solvent or:

4. 0.94 lbs-primary magnesium phosphate 1.88 lbs. mono basic ammonium hydrate 0.094 lbs. chromic acid anhydride 2.914 lbs. total weight of chemicals per gallon of aqueous solvent. The specific gravity of this solution at F. is 1.13.

The optimum dip solution when used to roll-coat sheets of electrical steel was found to be unsatisfactory, since among other things, it lacked the viscosity required for the deposition of an adequate film of chemicals. As a result, the coating applied was insutlicient to provide a suitable insulating film. I discovered that double quan tities of chemicals, see (2) of Tables 1 and II, could be used successfully for the rolling method to provide a good insulating coating film. The total weight of chemicals .in an optimum solution or" double the content of solution (1) of Table II is 0.62 parts in one part by weight of the aqueous solvent.

A solution with triple the quantity of weight of chemicals, see (3) of Table II, of the optimum solution was also prepared and found to remain in solution if the solution temperature was maintained at a minimum value of about 200 F. This concentration was not suitable for room temperature methods of coating applications, deposits a rather heavy film, and provides a baked coating of greater dusting characteristics. The solution concentration illustrates the maximum content possible with technical grade primary magnesium phosphate. This solution contains 0.93 parts of chemicals by weight and 1 part by weight of the aqueous solvent.

In all mixtures (l) to (3) of Tables I and II, the water is adjusted to make up 100 parts by weight of the aqueous solvent. In testing the effectiveness of my coating, electrical steel was prepared and given a complete evaluation. The coating showed superior lubricating qualities during punching or stamping runs. Such tests, when compared to those on the standard of bare steel lubricated with soapy water showed a considerable improvement in die life. The insulation value measured by standard tests indicated that the coating had an excellent resistance in the as coated condition and can be suitable for applications that need stress relief annealing at 650 C. in a mildly reducing air to cracked or natural gas mixture of a ratio of about 7 to 1. The thickness of the coating of the test sheets was about .0005 inch per side.

From the above discussion, it will appear that I have been able to provide three solution ranges which distinguish from each other by double or triple the quantities of each basic solute chemical employed. In this manner, the desired characteristics of the coating, as applied, are retained Without excessive dusting, lack of adhesion, etc., which would otherwise occur if, for example, the two salts were increased above their specified ranges without a proper proportionate increase in the chromic acid anhydride. The chromic acid functions as a solution reagent in this connection, although an excessive quantity will adversely affect the dusting and adhesion characteristics of the coating. The above information may thus be summarized, as follows:

In this table, the parts given are parts by weight of each solute in 100 parts by weight of an aqueous solvent.

If thus appears that with a constant weight amount of chromic acid there are many combinations possible: from 5 to 75 parts by weight of primary magnesium phosphate solute in 100 parts by weight of water solvent and to 135 parts by weight of mono basic ammonium phosphate solute in the 100 parts by weight of water solvent. The following represents optimum weights for each of the three solutions:

In this table, (1) a, (2) a and (3) a, are parts by weight of each solute in 100 parts by Weight of the aqueous solvent, while (1) b, (2) b and (3) b, are the percentages by weight of each solute in 131, 162 and 193 parts by weight, respectively, of solutions.

The water is adjusted in each mixture to make 100 parts by weight of solvent.

What I claim is:

1. A method of providing an improved insulating coating on the surface of electrical steel containing silicon which comprises, making up a water solution of all dis solved chemicals wherein the water solvent for the solution containing the following stated ranges of chemicals represents 100 parts by Weight and the dissolved solute chemicals constitute: about 5 to 75 parts by weight of primary magnesium phosphate, about 10 to 135 parts by weight of mono basic ammonium phosphate, and about 1 to 6 parts by weight of chromic acid anhydride; applying the solution as a film to the surface of the steel while the chemicals are in solution, and baking the film and forming an adherent coating on the surface of the steel.

2. A method as defined in claim 1 wherein the baking is efiected at a temperature of about 800 to 1000 F.

3. A method of providing an improved insulating coating on the surface of electrical steel containing silicon which comprises, making up a water solution of all dissolved chemicals wherein the water solvent for the solution containing the following stated ranges of chemicals represents 100 parts by weight and the dissolved solute chemicals constitute: about 5 to 25 parts by weight of primary magnesium phosphate, about 10 to 40 parts by weight of mono basic ammonium phosphate, and about 1 to 3 parts by weight of chromic acid anhydride; applying the solution as a film to the surface of the steel while the chemicals are in solution, and baking the film and forming an adherent coating on the surface of the steel.

4. A method of providing an improved insulating coating on the surface of electrical steel containing silicon which comprises, making up a water solution of all dissolved chemicals wherein the water solvent for the solution containing the following stated ranges of chemicals represents 100 parts by weight and the dissolved solute chemicals constitute: about to 50 parts by weight of primary magnesium phosphate, about 40 to parts by weight of mono basic ammonium phosphate, and about 2 to 5 parts by weight of chromic acid anhydride; applying the solution as a film to the surface of the steel while the chemicals are in solution, and baking the film and forming an adherent coating on the surface of the steel.

5. A method of providing an improved insulating coating on the surface of electrical steel containing silicon which comprises, making up a water solution of all dissolved chemicals wherein the water solvent for the solution containing the following stated ranges of chemicals represents parts by weight and the dissolved solute chemicals constitute: about 30 to 75 parts by weight of primary magnesium phosphate, about 60 to parts by weight of mono basic ammonium phosphate, and about 3 to 6 parts by Weight of chromic acid anhydride; applying the solution as a film to the surface of the steel while the chemicals are in solution, and baking the film and forming an adherent coating on the surface of the steel.

6. A method of providing an improved insulating coating on the surface of electrical steel containing silicon which comprises, making up a Water solution of all dissolved chemicals wherein the Water solvent for the solution containing the following stated ranges of chemicals represents 100 parts by Weight and the dissolved solute chemicals constitute: about 10 to 30 parts by Weight of primary magnesium phosphate, about 20 to 60 parts by weight of mono basic ammonium phosphate, and about 1 to 3 parts by Weight of chromic acid anhydride; applying the solution as a film to the surface of the steel while the chemicals are in solution, and baking the film and forming an adherent coating on the surface of the steel.

7. An improved inorganic composition for providing an insulating surface coating on electrical steel which consists essentially of about 5 to 75 parts by weight of primary magnesium phosphate, about 10 to 135 parts by weight of mono basic ammonium phosphate, and about 1 to 6 parts by weight of chromic acid anhydride, all as dissolved solutes in a solution containing water as the solvent and in parts by weight based on the Weight of 100 parts by weight of Water solvent.

8. An improved inorganic composition for providing an insulating surface coating on electrical steel which consists essentially of about 10 to 30 parts by weight of primary magnesium phosphate, about 20 to 60 parts by Weight of mono basic ammonium phosphate, and about 1 to 3 parts by weight of chromic acid anhydride, all as dissolved solutes in a solution containing water as the solvent and in parts by Weight based on the weight of 100 parts by weight of water solvent.

References Cited in the file of this patent UNITED STATES PATENTS 2,144,425 Cook Ian. 17, 1939 2,161,319 Schamberger June 6, 1939 2,348,698 Thompson May 9, 1944 2,357,269 Russell et a1. Aug. 29, 1944 2,413,949 Broverman Jan. 7, 1947 2,418,608 Thompson et a1 Apr. 18, 1947 2,472,592 Kiefer June 7, 1949 2,484,242 Hagel Oct. 11, 1949 2,501,846 Gifiord Mar. 28, 1950 2,554,250 Horstman et al. May 22, 1951 

1. A METHOD OF PROVIDING AN IMPROVED INSULATING COATING ON THE SURFACE OF ELECTRICAL STEEL CONTAINING SILICON WHICH COMPRISES, MAKING UP A WATER SOLUTION OF ALL DISSOLVENT CHEMICALS WHEREIN THE WATER SOLVENT FOR THE SOLTUION CONTAINING THE FOLLOWING STATED RANGES OF CHEMICALS REPRESENTS 100 PARTS BY WEIGHT AND THE DISSOLVED SOLUTE CHEMICALS CONSTITUTE: ABOUT 5 TO 75 PARTS BY WEIGHT OF PRIMARY MAGNESIUM PHOSPHATE, ABOUT 10 TO 135 PARTS BY WEIGHT OF MONO BASIC AMMONIUM PHOSPHATE, AND ABOUT 1 TO 6 PARTS BY WEIGHT OF CHROMIC ACID ANHYDRIDE; APPLYING THE SOLUTION AS A FILM TO THE SURFACE OF THE STEEL WHILE THE CHEMICALS ARE IN SOLUTION, AND BAKING THE FILM AND FORMING AN ADHERENT COATING ON THE SURFACE FOR THE STEEL. 